Inks have been used to mark items for a number of purposes, for example, for determining their authenticity or unique identification. Such markings, e.g., covert markings, may be used to identify goods that are intended for sale in a particular market. These markings are intended to prevent profit through the selling of goods for higher margins in a higher value market, where such goods were originally distributed for sale in a lower value market.
Covert marking applications typically require that the markings are both invisible to the human eye and difficult to reproduce in the event that the marking is discovered. Such marks can be produced through careful application of dyes that exhibit useful Stokes shifts when excited with a particular wavelength of light. UV dyes, for example, when excited with UV light will often emit a human detectable emission in the visible region.
Many lower cost materials that exhibit this phenomenon emit light in the blue region. This phenomenon may be useful, but has some serious drawbacks. Primarily, the ability to detect “UV marks” is quite easy because you only need a source of UV light (ubiquitous these days) and your eye, thus rendering the security of these marks as questionable. Secondarily, the effectiveness of blue emitting materials is limited, sometimes severely so if the substrate is white and has been treated with optical brighteners. Substrates treated with optical brighteners also emit blue light when excited with UV light thus making the contrast between substrate and mark difficult if not impossible to determine.
Infrared (IR) dyes have the benefit of excitation and emission in the infrared region. Without the aid of a carefully constructed viewing device, IR emission is not detectible to the human eye. A major drawback of these systems is that by their nature IR dyes are not particularly lightfast. Many efforts have been made to improve dye stability through dye structure manipulation and through favorable formulation development, i.e., inclusion of stabilization addenda.
It is known that inks for printing generally include a vehicle or solvent, a colorant (dye or pigment), a binder material (that affixes the colorant in place once the vehicle is adsorbed or evaporates) and other addenda that are specific to the printing methodology in use. In the case of solvent-based continuous inkjet printing, the vehicle may be a combination of organic solvents, the colorant (a dye or pigment depending on the equipment design and substrate requirements), a binder material (generally a polymer having sufficient solubility, viscosity, solvent release and binding properties), and conductivity improver (to enable sufficient charging of discrete ink droplets) and other addenda that enhance overall ink performance.
Polymethine (cyanine) dyes are one class of several possible classes that are nicely soluble in a variety of continuous inkjet printing systems and exhibit good fluorescence quantum yield. However most IR dyes, including polymethine dyes, are susceptible to undesired reactions leading to their possible depletion before an inks' desired lifetime has expired.
IR dyes are generally dark green to black when dissolved in solvent. It is a requirement that the dye be invisible to the eye when printed in the form of an ink (low or no stain). When printed, IR dyes can be visualized in an absorptive or emissive mode. Greater selectivity, and thus security, is enjoyed when the dyes are visualized in the emissive mode. This necessitates that the viewing device be constructed such that the excitation and emission wavelengths be sufficiently separated so that only the dye emission is detected and displayed. An additional requirement is that the dye be sufficiently diluted such that self quenching does not inhibit the emission process.
U.S. Pat. No. 5,093,147 discloses infrared fluorescing inks which are useful for printing invisible markings on the surface of an article. The inks use known polymethine (cyanine) laser dyes. Although the dyes used provide invisible markings, the cyanine dyes, unfortunately, have the disadvantage of fading or decomposing upon brief exposure to ultraviolet light.
It is known that near IR emissive dyes are susceptible to decomposition and that structural variation of the dye itself as well as the inclusion of protective addenda may lead to improved results. Said improvements are expensive and research intensive and do not completely eliminate dye fade. An apparative solution to mitigate the effects of dye fade through the design of the viewer is therefore desirable.
U.S. Pat. Nos. 7,712,667 and 7,516,899 disclose a hand held wireless reading viewer of invisible barcodes. It includes a display offering a live preview of the camera image. The intended function of this preview is only to find and center a non-human readable barcode. A microprocessor decodes the barcode and the result of the operation is the display and transfer of the barcode data.
U.S. Pat. No. 6,184,534 discloses a data reader and media bearing fluorescent indicia and the method of pulsing a light emitting diode. It is recognized that the fluorescent intensity of the dyes can be variable and it is suggested to pulse the light source with a current that is significantly higher than the steady state rated current in order to increase the amount of excitation light. The data reader is intended for decoding machine readable codes. It is shown positioned at a distance from the item with the fluorescent indicia with a substantial area of free space between the reader and the indicia. This allows ambient light to interfere with the signal from the fluorescent indicia.
U.S. Pat. No. 6,700,613 discloses a camera system for capturing both a visible and an invisible image. Like U.S. Pat. No. 6,184,534 the camera system is shown at a variable distance from the object without a means of excluding ambient light.
U.S. Pat. No. 7,357,326 discloses a fluorescent or luminescent optical symbol scanner intended for decoding a machine readable code. Exclusion of stray ambient light is important. This disclosure includes a shroud as part of the scanner. In order to realize the benefit of the shroud, however, the operator has to force the scanner onto the marked item.
U.S. Pat. No. 7,174,400 discloses thermal printing ribbons marked with an infrared fluorescent compound. For visualization of the markings, a hand held device is proposed where a camera is mounted inside a box above an opening at the bottom. Laser diodes are used to illuminate the sample and the image is viewed on a swivel monitor on top of the box.
None of the prior art recognizes that fading of the dye image is unavoidable and therefore additional measures are desirable to compensate for the fade of the dye using, for example, electronic means. The devices in the prior art are also commonly used to decode machine readable codes. An image preview function is only used to find the machine readable indicia, whereupon a microprocessor decodes the code. It is not recognized that human readable codes are an easier alternative in particular because they can be printed and read in a string that is wider than the field of view of the viewing device. Furthermore, the devices in the prior art are generally supported independently of the marked item, for example by holding a pistol grip-like manner, thus making it difficult to shield ambient light, in particular when the marked item lies flat.